49-31-00
 ALL  ú ú 09 Page 6 ú Jul 12/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
CONTROL AIR CONNECTION
ACCELERATION HIGH-PRESSURELIMITER FUEL PORTVALVE
FUEL FILTER FUEL GOVERNOR
FUEL 
CONTROL UNIT 
FUEL  PRESSURE 
DRAIN  PORT FOR 
PORT  THE FUEL 
INLET 

LEGEND CONTROL AIR DRAIN HIGH-PRESSURE FUEL LOW-PRESSURE FUELCOMBUSTOR UNIT
FLOW
DIVIDER
VALVE
FUELFUEL SOLENOID ATOMIZER
VALVE COMPRESSOR
HIGH-PRESSURE SECTIONFUEL FILTER IGNITER
PLUG
COMBUSTION CHAMBER
TURBINE
SECTION
TURBINE FUEL
EXHAUST PUMP
APU CONTROL THERMOSTAT
COMPRESSOR AIR INLET GEARBOX FUEL DRAIN
ACCESSORY
CONNECTION PORT 3-WAY
FUEL INLET SOLENOID VALVE
LOW-PRESSURE
FUEL FILTER
DIFFERENTIAL PRESSURE FUEL HEATER
REGULATOR BLEED AIR OUTLET
CHECK VALVE RATE CONTROL VALVE
FUEL 
SUPPLY  BLEED AIR  SWITCHER  SOLENOID 
VALVE  VALVE 
PNEUMATIC 
ACTUATOR 

APU Fuel System Schematic
Figure 2 (Sheet 1)

EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITHOUT ETC ú

 49-31-00
 ú ú 01 Page 7 ú Mar 15/92
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES /MAINTENANCE MANUAL ////////////////////////
CONTROL AIR CONNECTION
HIGH-PRESSURE FUEL PORT
ACCELERATION LIMITER VALVE
FUEL SOLENOID VALVE
FUEL GOVERNOR
FUEL 
CONTROL UNIT 
FUEL  PRESSURE 
DRAIN  PORT FOR 
PORT  THE FUEL 
INLET 
FUEL HEATER 

CHECK VALVE
DIFFERENTIAL PRESSURE REGULATOR 3-WAYSOLENOID VALVE
COMBUSTOR
UNIT
FLOW 
DIVIDER 
VALVE 
FUEL 
HIGH-PRESSURE  COMPRESSOR  ATOMIZER 
FUEL FILTER  SECTION 
IGNITER 
FUEL PUMP  PLUG CHAMBER COMBUSTION 
TURBINE 
SECTION 
TURBINE 
EXHAUST 
ETC 
THERMOCOUPLE 
CHECK VALVE 
FOR THE 
COMPRESSOR AIR INLET  TURBINE DRAIN 
ACCESSORY GEARBOX  LEGEND  CONTROL VALVE PROPORTIONAL 
CONNECTION  CONTROL AIR 
PORT FOR 
THE FUEL  DRAIN 
INLET  HIGH-PRESSURE FUEL  INLET 
LOW-PRESSURE FUEL  TEMP 
FUEL FILTER LOW-PRESSURE  ETC  GAUGE EGT 
BLEED 
FUEL  AIR OUTLET 
SUPPLY 
BLEED AIR  SWITCHER  SOLENOID 
VALVE  VALVE 
PNEUMATIC 
ACTUATOR 

RATE CONTROL VALVEORIFICE TEST PORT
APU Fuel System Schematic Figure 2 (Sheet 2)
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITH ETC ú

 49-31-00
 ú ú 01 Page 8 ú Jul 15/93
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
C.  AIRPLANES WITH THE ETC; The exhaust gas temperature is controlled by the electronic temperature control (ETC). For a more detailed description of the ETC, refer to AMM 49-61-00/001.
 D.  AIRPLANES WITHOUT THE ETC; The exhaust gas temperature (EGT) is controlled by the APU control thermostat. The APU control thermostat has two functions:
 (1)  
The APU control thermostat is an acceleration control thermostat at an APU speed up to 95%. When the EGT is higher than 620-649°C (1150-1200°F), the APU control thermostat opens to bleed control air from the acceleration limiter valve. This permits the bypass valve of the fuel control unit to send fuel back to the inlet fuel port and will decrease the EGT. When the EGT decreases below the specified limit, the APU control thermostat closes. When the APU control thermostat closes, the bleed air from the acceleration limiter valve stops and the bypass valve closes.
　
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